Although the present invention can be applied to a variety of vertical insulated gate devices, an IGBT will be used as an example.
One type of common vertical IGBT contains a vertical pnp bipolar transistor (formed of many cells in parallel) which is driven by a MOSFET that begins the injection of carriers, which then fully turns on the pnp transistor. At high current levels, the forward voltage drop in a vertical IGBT (Vce-sat) is typically lower than that of a vertical MOSFET. In high power IGBTs that handle high currents and high voltages, such as for industrial motor control, induction heating, etc., the n-type base needs to be relatively lightly doped to create a wide depletion layer to withstand the high voltage in the off-state. This thick and lightly doped n-type base layer is a bottleneck to lowering the Vcs-sat. When the transistor is switched from on to off, it is important to quickly remove holes from the n-type base to rapidly stop the current flow.
A thinner, more heavily doped p-type emitter (which may be the bottom semiconductor layer of the IGBT) desirably increases hole injection efficiency and reduces the Vce-sat of the IGBT. There is, however, a trade- off between increased injection efficiency and the turn off speed of the IGBT. A thin p-type emitter layer is typically formed after completing the front end processes (i.e., after forming the transistor layers), where the bottom surface of the wafer (a silicon substrate) is mechanically ground down, followed by the p-type emitter dopants being implanted in the bottom surface, followed by an anneal step. A laser anneal is desirable, since the resulting heat does not cause any of the front-side dopants to further diffuse. This annealing step adds to the process complexity and requires specialized equipment.
It is desirable for the device to have a high breakdown voltage but a low Vce-sat. Breakdown (in the off-state) in the area of the active cells can cause permanent device failure, so it is desirable for the device to provide a breakdown path away from the delicate active cell array. Efficient handling of high voltages without damage is generally referred to as robustness.
Areas near the edges of the die are particularly susceptible to breakdown due to electric field crowding and should not be a bottleneck to the overall breakdown voltage of the device. Termination structures are typically used around the edges of the die for high power devices.
Improvements in all of the above-mentioned areas are desirable to create a more robust and efficient IGBT.